Problems with sensors and noisy readings are more likely to be caused by voltage transients (such as lighting) than by poor quality or poor installation techniques. Transient protection modules, when combined with other measures, can help prevent these problems.
Transients and Buried Sensors
The drawing below shows lightning striking a tree. You can also see a sensor installed in a borehole. The concentric lines are iso-voltage levels at the instant of the strike. Each line represents a voltage difference of about 50 kV. The cable to the sensor cuts across voltage levels, so there is about a 10 to 20 kV voltage gradient between the surface and the sensor.
Sensors and cables cannot withstand such high voltage differences. The typical result is arcing that can destroy the sensor and cable. Even if the sensor is not destroyed, the cable jacket will have pinhole punctures. Water will enter through these punctures later, causing all the symptoms of a seal failure and a significant increase in noise and interference.
Protecting Buried Sensors
To protect buried sensors, we must try to eliminate voltage gradients that would affect the sensor and cable. In the drawing above, the sensor cable is terminated at the surface and we must eliminate any gradient developing between the surface and the sensor. This can be achieved by running a heavy gauge, bare grounding wire down the borehole, parallel to the sensor signal cable. The wire should extend several feet deeper than the sensor and should be separated by an insulator where it passes the sensor body.
When signal cable is not terminated at the top of the borehole, but instead runs horizontally to a readout station or data logger, the simple grounding wire technique above is no longer effective. Now we must try to eliminate the voltage gradient between the readout station and the sensor.
A cost-effective way to eliminate this gradient is to place a transient protection module at the top of the borehole, between the horizontal and vertical runs of signal cable. The grounding lug of the protection module is then connected to the grounding wire in the borehole.
With the protection module in place, transients on the horizontal run of signal cable are intercepted and shunted to the grounding wire in the borehole. This instantly raises the voltage levels around the sensor and signal cable, eliminating the voltage gradient that causes damage.
Note that the transient protection suggested above does not protect the horizontal runs of signal cable. Horizontal runs can be protected only by a grounding wire, grid, pipeline or other conductor that runs parallel to the cable over its full length.
Protecting Data Loggers
Transient protection modules placed between signal cable and the data logger can protect the logger from transients arriving on the signal cable. In this case, the grounding lug on the module should be connected to the local facility ground, probably the same ground that the data logger is connected to. Other measures to protect surface equipment include the use of isolated power supplies and optical isolation of communications lines.
For more information
You may also be interested in papers on this subject:
Shoup, Dale: Sensors in the Real World , Protecting Geotechnical Sensors and Cable from Lighting Damage. This paper is the theoretical basis for the transient protection steps suggested above. The paper concentrates on buried sensors and presents some case studies.
Moulds, A and Watson, A: Lightning Protection for Dam Instrumentation , a Case Study of the New Victoria Dam. The authors used concepts from Dale Shoup above in conjunction with a six point plan developed by Erico, a manufacturer of transient protection devices for (surface) facilities.